As electronic devices such as notebook computers, mobile phones and digital cameras have widely spread, the demand for secondary batteries for driving these electronic devices has increased. Recently, in these electronic devices, the power consumption increases in association with more advanced functions and miniaturization is expected, and thus improvement in performance of secondary batteries is demanded. Among secondary batteries, a nonaqueous-electrolyte secondary battery (particularly, lithium ion secondary battery), which can be designed to have a higher capacity, is actively applied to various electronic devices.
A nonaqueous-electrolyte secondary battery generally has such a configuration that a positive electrode in which a positive electrode active material layer having a positive electrode material represented by a positive electrode active material is formed on the surface of a positive electrode collector, and a negative electrode in which a negative electrode active material layer having a negative electrode material is formed on the surface of a negative electrode collector are connected via a nonaqueous-electrolyte (nonaqueous electrolytic liquid), and accommodated in a battery casing.
In a lithium ion secondary battery, which is a representative example of a nonaqueous-electrolyte secondary battery, a complex oxide of lithium is used as a positive electrode material (positive electrode active material). The complex oxide is described, for example, in Patent Literatures 1 to 6.
Patent Literature 1 describes employing a positive electrode active material formed by mixing LixCoMO2 and LiNiMnMO2 (M: selected from predetermined elements). The positive electrode active material has an active material showing high average voltage at the time of discharging, and an active material having high thermal stability.
Patent Literature 2 describes employing a positive electrode active material containing a crystal structure of a layered rocksalt-type structure of LiNiMnTiO2. The positive electrode active material, which contains Ti, can afford a higher charge and discharge capacity in comparison with the case not containing Ti.
Patent Literature 3 describes employing a positive electrode active material formed by mixing LixMnMO4 and LiNiMO2 (M: selected from predetermined elements in both cases). The positive electrode active material achieves excellent battery performance after storage at high temperature.
Patent Literature 4 describes a positive electrode active material in which part of Li in LiMnMO2 having a layered polycrystalline structure (M: selected from predetermined elements in both cases) is defined. In the positive electrode active material, distortion in crystals and chemical bonds are stabilized, and the effects such as cycle stability at the time of charging and discharging, and durable stability are achieved.
Patent Literature 5 describes a positive electrode active material in which each of part of Li and Co in LiCoO2 is substituted by a predetermined element M (M: selected from predetermined elements in both cases). In the positive electrode active material, by substituting each of Li and Co by an element M, the binding force between the lithium layer and the cobalt layer is enhanced and distortion between these layers and expansion of crystal lattice are controlled, resulting that the effects such as cycle stability at the time of charging and discharging, and durable stability are obtained.
Patent Literature 6 describes employing a positive electrode active material in which LiNiMnCoO2 and Li2MO3 are mixed (M: selected from predetermined elements). The positive electrode active material has an active material that exerts excellent effects in terms of battery capacity and safety, and an active material that exerts effects in terms of cycle characteristics and storage characteristics.
However, in any of these positive electrode active materials (positive electrode materials), collapse of the crystal structure at the time of charging and discharging cannot be sufficiently controlled. This disadvantageously leads deterioration in the capacity of the nonaqueous-electrolyte secondary battery. Also regarding the safety, the technique of employing a positive electrode containing Ti, namely a positive electrode formed of LiNiMnTiO2 is described in Non-Patent Literature 1. However, as described in Non-Patent Literature 1, it is described that addition of Ti does not result in overwhelming improvement in safety.
As another attempt to achieve both the safety and the high stability of crystals, a technique of employing a positive electrode containing Si having strong binding force with oxygen in the same amount with transition metal, namely a positive electrode formed of Li2MnSiO4 is described in Non-Patent Literature 2. However, in this positive electrode, since the transition metal assumes a four-coordination structure, the structure is destabilized at the time of charging, and the positive electrode does not have sufficient durability.
Further, Patent Literature 7 describes an electrochemical active material that is obtained by substituting part of nickel in a single-phase complex oxide of nickel and lithium in the form of LiNiO2. Specifically, a single-phase oxide satisfying Li(M1(1−a−b−c)LiaM2bM3c)O2(0.02<a≤0.25, 0≤b<0.30, 0≤c<0.30, (a+b+c)<0.50, M3: at least one element selected from Al, B, and Ga, M2: at least one element selected from Mg and Zn, M1=Ni(1−x−y−z)CoxMnyM4z, M4: at least one element selected from Fe, Cu, Ti, Zr, V, Ga, and Si, 0≤x<0.70, 0.10≤y<0.55, 0≤z<0.30, 0.20<(1−x−y−z), b+c+z>0) is described.
Patent Literature 8 describes a Li oxide satisfying xLiMO2·(1−x)Li2M′O3 (0<x<1, M: three or more ions including Mn, Co, and Ni, Mn:Ni=1:1, Mn:Co=1:1, M′:Mn) and a battery.
Patent Literature 9 describes a thin-film battery employing Li—V oxide represented by LixV2Oy (0<x≤100, 0<y≤5) is described. As a positive electrode active material of this battery, LiCoO2, LiNiO2, LiMn2O4, LixMn2−yO4 (1.2<x<2.2, y=0.3), LiFePO4, LiVOPO4, LiTiS2, LiMnCrO4, LiCo1-xAlxO2 (0≤x≤1), V2O5, V6O13, VO2, MnO2, FePO4, VOPO4, TiS2, or MnO0.5Cr0.5O2 is described.